1. Field of the Invention
The present invention relates to a driving method for a liquid crystal display and a driving circuit thereof, and more particularly, to a driving method for liquid crystal display and a driving circuit thereof using a liquid crystal having bistable twisted nematic property.
2. Description of the Prior Art
Generally, a aligning direction of a liquid crystal inserted between upper substrate and lower substrate of a liquid crystal display is varied by an externally applied field because permittivity of long axis and permittivity of short axis are different, with respect to molecular axis, in terms of material property.
Particularly, a bistable twisted nematic (BTN) liquid crystal is the liquid crystal with cholesteric nematic phase having two metastable status which has 0xc2x0 and 360xc2x0 by adding a chiral dopant into a nematic liquid crystal. Such liquid crystal display using the bistable twisted nematic liquid crystal (BTN LCD) is better for realizing high resolution and dynamic picture images than the liquid crystal using nematic liquid crystal since low voltage driving is possible and switching speed also can be increased, and capable of providing wide viewing angle and excellent contrast ratio.
Recently many methods for effective driving of BTN LCD having generally superb characteristics have been disclosed.
In U.S. Pat. No. 5,594,464, a driving method for BTN LCD is disclosed, and a method of applying selection pulse to a liquid crystal cell after applying reset pulse predetermined voltage to the liquid crystal cell for predetermined period is used, as shown in FIG. 4a. 
A driving method enhancing switching speed of response of liquid crystal is disclosed in Korean patent application No. 98-852 filed by the same applicant of the present invention, and is to apply selection pulse after applying reset pulse for predetermined period while having predetermined idle time before applying selection pulse.
In the two methods, reset pulse and selection pulse applied to the liquid crystal are applied via alternating voltage having zero accumulation value of DC voltage per frame (DC CHARGE) for DC FREE of the liquid crystal.
That is, to apply reset pulse of negative voltage xe2x88x92Vreset, opposite of positive voltage, right before applying reset pulse of positive voltage +Vreset for the same period time, and to apply negative voltage xe2x88x92Vselection, opposite of positive voltage, as selection pulse right before applying positive voltage +Vselection in a frame, which make the effective voltage applied to the liquid crystal in a frame zero.
To apply these four levels, +Vreset, +Vselection, xe2x88x92Vreset, and xe2x88x92Vselection, of voltage to the liquid crystal, conventionally, a method in which the output voltage, scan signal, of common driving element which is a voltage applying mean and output voltage, data signal, of segment driving element are alternated to opposite potential level every certain period is used.
To realize such conventional way of driving method for the liquid crystal display, a driving circuit having multiplexers which switch the four levels of voltage each period and output pulse to a input terminal of the common driving element and segment driving element is used.
The switching speed of multiplexer can be increased if the multiplexer is configured to drive positive voltage rather than negative voltage with regard to the characteristics of a semiconductor device, but in conventional way there is a restriction of increasing driving speed since negative voltage is also needed for driving the liquid crystal.
In addition, since a signal switched by the multiplexer has large voltage margin which is occurred when it is changed from negative level to positive level, there is a problem that a flicker in visual might happen on a display screen.
The present invention has been made in an effort to solve the above described problems of the prior art.
The objective of the present invention is to apply necessary scan signal and segment signal in one of potential levels and make an effective voltage applied to both ends of actual liquid crystal cell zero within certain frame.
To achieve the above objective, the present invention provides a driving method for a liquid crystal display forming reset pulse and selection pulse applied to a liquid crystal cell by the difference of potential level between a first signal and a second signal which are output from a fist driving element and a second driving element, wherein the driving method is characterized in that though the first signal and the second signal are kept in one of positive and negative potential levels, and applied to the liquid crystal cell, an effective voltage applied to actual liquid crystal cell becomes zero and DC FREE is realized.
In the present invention, the first signal and the second signal are respectively applied in two frames, a first frame and a second frame, though a signal applied in the second frame is formed by inverting waveform of a signal applied in the first frame, but by making a waveform portion of maximum value in the second frame correspond with a waveform portion of minimum value in the first frame, the signal of the first frame constituted from a reset pulse and a selection pulse is inverted to reverse polarity in the second frame and the effective voltage value during these two frames becomes zero.
And the liquid crystal cell adopts bistable twisted nematic liquid crystal in cholesteric nematic group having two metastable status.
It is another objective of the present invention to provide a driving circuit for a liquid crystal display which is to realize above driving method.
The driving circuit which applies the first signal and the second signal to the liquid crystal cell so that a aligning angle of liquid crystal molecules are changed by the field resulting from the difference in potential level between the two signals, comprises:
a voltage applying means for applying the first signal and the second signal,
a control signal applying means for generating a switching control signal and a level-inverting switching control signal, and
a multiplexer which selectively switches the first signal and the second signal in accordance with the switching control signal, or inverts and switches the first signal and the second signal in accordance with the level-inverting switching control signal and outputs them to a driving element connected to the liquid crystal cell.